The invention relates to a display device comprising an electro-optical medium between two supporting plates which are located opposite each other and whose facing sides are provided with a plurality of column electrodes and row electrodes, respectively, while at least one of the supporting plates is provided with a plurality of picture electrodes which are connected to the row or column electrodes via non-linear switching elements, each non-linear switching element comprising a non-linear resistive material between a first and a second electrically conducting layer.
Display devices of this type are used, for example as display panels in measuring equipment, personal computers and television receivers.
The invention also relates to a non-linear switching element and to a method of manufacturing a non-linear switching element.
These non-linear switching elements may not only be used in said display devices but also, for example in circuits for pressure sensors or photosensitive panels.
A display device of this type is known from European Patent Application EP-A-0 202 092. The display device shown in this Application comprises a non-linear resistive element with a layer of non-linear resistive material, for example silicon oxide or silicon nitride between a conducting layer and a metal electrode. The metal electrode forms part of the row or column electrodes.
It has been found that the switching elements may exhibit a given leakage current after a period of time at a voltage of 0 V (or a small voltage which is much smaller than the threshold voltage). The picture elements associated with the switching elements lose their charge due to such a leakage current and the written picture contents are not maintained. The effective lifetime of these types of switching elements may thus be limited to 100 hours.
It is an object of the invention to obviate the above-mentioned problem as much as possible.
To this end a display device according to the invention is characterized in that the electrically conducting layer located at the side of the electro-optical medium is metallic and is separated from the non-linear resistive layer by a metallic protective layer.
The invention is based on the recognition that, upon later deposition of a conducting layer by means of, for example sputtering, the presence of the protective layer prevents locations on the surface of the layer of non-linear resistive material from being damaged in such a way that leakage currents occur at a later stage. To this end the metallic protective layer should be free from pinholes and discontinuities. This can be achieved by means of low-energetic deposition techniques such as, for example sputtering.
An electrically conducting metallic layer can then be provided on the protective layer, which metallic layer may be thicker and may be made of a low-ohmic material so that a satisfactory contact can be realised. Due to the presence of the protective layer, a sputtering energy or a sputtering rate which are higher than those for the protective layer can be used when this metallic conducting layer is being provided.
By providing the electrically conducting layer at a higher sputtering rate (for example, a factor of 5-10 higher), the manufacture may proceed more rapidly. For protecting the underlying layer of non-linear resistive material it is, however, sufficient that this material is contacted by means of a metallic layer which is obtained by using low-energetic deposition techniques. A further display device according to the invention is therefore characterized in that the electrically conducting layer located at the side of the electro-optical medium is provided by means of a low-energetic deposition technique.
If the protective layer comprises a high melting point material such as, for example molybdenum or a molybdenum alloy (for example, Mo--Ta, Mo--Ti, Mo--Si, Mo--Cr, Mo--V, Mo--Nb, Mo--W, Mo--Ta--Ti, Mo--Ta--Si, Mo--Ta--V, etc.), material of this protective layer is prevented from diffusing into the layer of non-linear resistive material, for example during subsequent process steps. The thickness of the protective layer is preferably at least 10 nm so as to prevent the occurrence of discontinuities and pinholes in the protective layer. The thickness is further dependent on the sputtering energy which is used when the conducting layer is being provided and is preferably chosen to be smaller than 100 nm (and 80 nm at lower sputtering energies).
The protective layer and a layer of conducting material which is deposited thereon may consist of the same material.
If the protective layer consists of a high melting point material, low melting point materials such as, for example aluminium, copper, silver and nickel-chromium, may alternatively be used for the conducting layer. In particular, low melting point well-conducting metals can be used which would diffuse into the nonlinear resistive material at a higher temperature during later process steps if the protective layer of high melting point metal were not used.
A first method of manufacturing, on a substrate, a non-linear switching element provided with a layer of non-linear resistive material between a first and a second electrically conducting layer is characterized in that after the provision of the first electrically conducting layer on the substrate
1) a layer of the non-linear resistive material is deposited on the substrate and on the first conducting layer; PA1 2) a metallic protective layer is provided on the layer of non-linear resistive material; PA1 3) the assembly is provided with a first mask; PA1 4) the metallic protective layer and the layer of non-linear resistive material are patterned by means of the first mask; PA1 5) a second layer of electrically conducting material is deposited on the assembly thus obtained at a deposition energy which is equal to or larger than that which is used for depositing the metallic protective layer, whereafter a second mask is provided, and PA1 6) the second layer of electrically conducting material is patterned by means of the second mask.
In a second method the metallic protective layer and the layer of electrically conducting material are deposited directly after each other on a layer of patterned non-linear resistive material and subsequently jointly patterned.
In a further method only a metallic protective layer is provided by means of low-energetic deposition techniques.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.